1. Field of the Invention
The present invention relates generally to electrophotographic apparatus and, more particularly, to an electrophotographic apparatus in which solid developers of a plurality of colors which are solid at room temperature and changed into liquid when heated are heated by a predetermined heater and changed into liquid and supplied to an electrostatic latent image so that the electrostatic latent image is developed as a multi-colored printed image.
2. Description of the Prior Art
In an electrostatic process such as an electronic photograph developing process or the like, a photosensitive material such as a photoconductor or the like is uniformly electrified and selectively illuminated by a laser beam. Then, charges on the portion illuminated by the laser beam are extinguished to form an electrostatic latent image. Alternatively, a dielectric material such as a paper, a plastic film or the like is electrified by an electrostatic electrode called a multi-stylus head in response to an electrical signal, thereby forming an electrostatic latent image. In order to develop this electrostatic latent image, a toner or developer charged to the polarity opposite to that of an electrostatic latent image carrier (photoconductor drum, dielectric film and so on) having the electrostatic latent image is electrostatically deposited on the electrified portion of the photoconductor drum and then developed.
As a developing method, a dry type developing system using a dry developer and a wet type developing system using liquid developer are known. The dry developers are generally formed of very small particle powders. For this reason, if the dry developers are scattered, a problem of environmetal disruption occurs. To solve this problem, a developer cartridge in which dry developers are accommodated should be constructed as a sealed type. Today, most of the developing apparatus are of such a type that an electrostatic latent image carrier and a developing portion are wholly exchanged. Therefore, this type of developing apparatus is expensive but the dry developer is excellent in preservation and if the developers are accommodated within the developer cartridge, they are easy to handle.
On the other hand, the liquid developer is formed by dispersing into insulating liquid particle powders of colorant such as dye stuff and the like. By a centrifugal pump or the like, the liquid developer is injected from the developer container through a slit of a developing electrode used to charge the electrostatic latent image to the polarity opposite to that of the electrostatic latent image carrier, whereby colorant particle powders are electrostatically deposited on the electrostatic latent image. In the conventional developing apparatus, extra liquid developers, which are not deposited on the electrostatic latent image, are returned to and accommodated again within the developer container, rendering the concentration of colorant particle powders in the liquid developers low. This makes control of the concentration of the developing liquid (liquid developer) difficult. Further, various problems arise such as that pollution occurs because of the leakage of liquid developer in the developing process or when the developing apparatus is held or when the liquid developer is exchanged and that the preservation of liquid developer is difficult because the colorant particle powders in the liquid developer tend to coagulate and precipitate. The electrostatic process utilizing the liquid developer offers the possibility that the resolution and gradation of the picture will be increased to levels equal to those of a silver halide photograph. Therefore, this electrostatic process is expected as an electrostatic latent image developing system which is suitably applied to a printing apparatus of high image quality such as a video printer used in an electronic still camera or the like.
In the background set forth so far, the assignee of the present application has previously proposed a method of developing an electrostatic latent image (see Japanese Patent Application No. 63-156847) and corresponding U.S. application Ser. No. 07/609,027 filed Oct. 30, 1990, which is a continuation of application Ser. No. 07/366,863 filed June 15, 1989. This previously-proposed method can solve various problems that the liquid developer is difficult to handle, the liquid developer tends to be smudged, the maintenace of liquid developer is difficult and the preservation of liquid developer is poor while making effective use of advantages of the electrostatic process using the liquid developer. In other words, this electrostatic latent image developing method is characterized in that a developer (i.e. toner) in which a colorant is dispersed into an electrostatic insulating organic material solid at normal temperature is heated and changed into liquid and an electrostatic latent image is developed by the thus liquefied developer in a dry developing fashion.
In the conventional method as described above, however, considering an electrophotographic apparatus capable of printing a video image in a plurality of colors, for example, three colors such as yellow (Y), magenta (M) and cyan (C), three containers for solid developers of three colors are required, which makes the electrophotographic apparatus complicated and expensive.